The invention relates to improvements in cutting tools, and more specifically, to improvements in cutting tools and shanks for machining and drilling plastics of various types, particularly acrylics and polycarbonates, as well as other non-metallics, and non-ferrous sheet metals.
Acrylic plastics are a family of plastics that rival glass in optical transparency. The primary advantage of the material is that it is many times more impact resistant than glass, and, unlike glass, it is readily fabricated with standard metal-working tools and equipment. Despite this, conventional tools leave much to be desired in the quality of work produced. While the material is initially transparent, after any machining operation, the material is left translucent or diffuse. Restoring an optical finish to the part requires progressive polishing operations. Internal surfaces can be extremely difficult or impossible to polish effectively.
Drilling is one of the most difficult operations in this heat-sensitive material, since a large amount of material is removed in a single pass. This generates considerable heat and chips which cannot be effectively dissipated, as the material is a poor thermal conductor and is inaccessible to external cooling. In addition, drill bits will characteristically produce a hole that is smaller than the size of the tool, because of the material's poor thermal conductivity, and its high thermal coefficient of expansion. This makes obtaining a precise size hole difficult, requiring either a special oversize tool, or secondary operation such as boring, or reaming and polishing. Moreover, these tools will produce a "bullet hole" exit upon breaking through the work. Industry's answer to this has been to use an acute drill point and/or to place flats on the cutting lips, to create a scraping action. Nonetheless, considerable care must be exercised, and clean holes are not necessarily assured. On critical work, back-up material is often used to prevent chipping on break-through. This is also a problem with milling cutters and turning tools which chip the work as they traverse off the part. Depth of cut and feed rate must be carefully controlled to avoid this. Furthermore, the aforementioned machining operations, especially drilling, leave residual stress in the workpiece. The stresses left in the material can later lead to crazing (spider-web like cracks) or even failure of the part. To insure against this, parts are annealed, i.e., exposed to elevated temperatures for a considerable period of time and allowed to cool slowly.
The invention described herein, overcomes all the shortcomings of conventional tooling and offers additional advantages.